This invention relates to a process for the production of chiral cis-5-amino-2-cyclopenten-1-ol as its cyclic carbamate.
cis-5-Amino-2-cyclopenten-1-ol is an extremely useful synthon, especially in the form of derivatives such as the cyclic carbamate 1 (3,3a,4,6a-tetrahydro-cyclopentaoxazol-2-one). For example, conversion of 1 to N-Boc derivative gives an intermediate which is a suitable substrate for Pd0-catalysed nucleophilic allylic displacement (Muxworthy et al., Tetrahedron Letters, 1995, 36, 7539-7540), as shown in the following Scheme. 
This reaction was demonstrated for 2 model nucleophiles. Products of such reactions establish 1,3-chiral centres on the cyclopentane ring. This structural motif is an important synthetic target, particularly for elaboration into carbocyclic nucleosides and analogues of nucleoside derivatives, reviewed for example by Agrofoglio et al. (Tetrahedron, 1994, 50, 10611-10670).
As further demonstration of the synthetic utility of the N-Boc derivative of compound 1, Anderson et al. (J. Org. Chem., 1998, 63, 7594-7595) report diastereoselective allylic amination reactions.
Synthesis of the cyclic carbamate 1 represents a challenge, especially when it is required in the form of a single enantiomer. Two syntheses of the latter have been reported. In the earlier of these, Muxworthy et al. (as above) used a mandelic acid derivative to introduce chirality. Thus an acyl nitroso intermediate, generated in situ from (R)-xcex1-hydroxyphenylacetohydroxamic acid, undergoes a Diels-Alder reaction with cyclopentadiene, and the resulting adduct is treated with dilute HCl to effect rearrangement to give a salt derivative of cis-5-amino-2-cyclopenten-1-ol. Further treatment with either Et3N/TsCl or t-BuOK/PhNTf2 yields 1 in modest yield (30-35%) and requires chromatographic purification. Overall, this route appears impractical to operate at scale, because of the extremely low temperature required in the initial cycloaddition. In addition, a stoichiometric amount of the expensive (R)-mandelic acid is required as chiral auxiliary, recovery and reuse of which is precluded by its mode of removal. 
In the second route, Mulvihill et al. (J. Org. Chem., 1998, 63, 3357-3363) employed a four-step sequence from cyclopentadiene, comprising acyl-nitroso cycloaddition, Nxe2x80x94O bond scission, bioresolution and rearrangement of functional groups. The requirement for excess Mo(CO)6 in the second step renders the overall approach unscaleable. 
The carbocyclic nucleoside abacavir, a potent reverse transcriptase inhibitor, is synthesised using enantiomerically pure 2-azabicyclo[2.2.1]hept-5-en-3-one as a chiral building block. This drug has been produced at multi-tonne scale, and an economical bioresolution of 2-azabicyclo[2.2.1]hept-5-en-3-one has been developed to provide the chiral building block as a single enantiomer (WO-A-98/10075). This uses a cloned lactamase at high volume efficiency: 
Both the residual lactam and the product amino acid can be converted into compounds of the following structures (2 and 3) using standard chemical methods. 
Maier et al (Synlett, August. 1998, 891-3) disclose the synthesis of cyclohexenylaminesby ring-closing metathesis. Oneproductofsuch reaction is further reacted as follows 
This invention is based on the discovery of an economical and convenient process to the cyclic carbamate 1. This process can be operated at scale, to access single enantiomer forms of 1 from starting materials 2 and 3 which are readily available in quantity via bioresolution of 2-azabicyclo[2.2.1]hept-5-en-3-one, and of which the simplest embodiment may be represented as follows: 
Halolactonisation of unsaturated carboxylic acids or their salts with iodine to form lactones has been known for several decades, and is a well-understood process. It was surprising that, when the cyclopentene carboxylic acid 2 was treated with iodine in a biphasic system of diethyl ether and saturated sodium bicarbonate, the expected xcex3-lactone 4 was not formed. Instead, the NHBoc function cyclises onto the adjacent carbon, to generate a putative intermediate 5 which undergoes decarboxylative elimination under the mildly basic conditions to give compound 1. The latter is obtained directly in high chemical purity. This is a scaleable reaction, and represents a superior route to the routes described above.
Instead of iodine, another halogen may be used. If required, a different carbamate protecting group can be used instead of N-Boc in the process of the present invention. The process may also be applied to N-substituted derivatives containing Nxe2x80x94R1 instead of Nxe2x80x94H, R1 being an organic group (alkyl, aryl, aralkyl etc, optionally substituted) of up to 20 C atoms. Also, the CO2H group may instead be CO2R2, R2 being H, C1-10 alkyl, aralkyl or aryl, e.g. of up to 10 C atoms.
In a preferred embodiment, the base is an aqueous alkaline base, preferably aqueous sodium carbonate, and the reaction is conducted in a biphasic system. Preferably, enantiomerically enriched carbamate 1 of at least 80% ee, and more preferably at least 95% ee, is prepared.
If not already protected, the product may be converted, by known means, to a protected, e.g. the N-Boc, derivative. This is then ready for processing to a desired final product.